Despite decades of dedicated efforts there are still basic questions to answer with regard to Supernova progenitor systems and explosion mechanisms. In particular, in the last years a number of exceptionally bright objects and extremely faint events
have demonstrated an unexpected large Supernova variety. The large number of Supernovae candidates at different redshifts provided by the next generation surveys, from ground and space, will allow to reach a better insight of the Supernova events in all their flavours. In particular it will be the possible to assess the systematics of type Ia Supernovae as distance indicator at any redshift. The Gaia astrometric mission is expected to discover a huge number of transient events, including Supernovae, which will be immediately disseminated to the astronomical community by a transients alert system for a suitable follow up.
The final fate of massive stars depends on many factors, including mass, rotation rate, magnetic fields and metallicity. Theory suggests that some massive stars (initially greater than 25-30 solar masses) end up as Wolf-Rayet stars which are deficien
t in hydrogen because of mass loss through strong stellar winds. The most massive of these stars have cores which may form a black hole and theory predicts that the resulting explosion produces ejecta of low kinetic energy, a faint optical display and a small mass fraction of radioactive nickel(1,2,3). An alternative origin for low energy supernovae is the collapse of the oxygen-neon core of a relatively lowmass star (7-9 solar masses) through electron capture(4,5). However no weak, hydrogen deficient, core-collapse supernovae are known. Here we report that such faint, low energy core-collapse supernovae do exist, and show that SN2008ha is the faintest hydrogen poor supernova ever observed. We propose that other similar events have been observed but they have been misclassified as peculiar thermonuclear supernovae (sometimes labelled SN2002cx-like events(6)). This discovery could link these faint supernovae to some long duration gamma-ray bursts. Extremely faint, hydrogen-stripped core-collapse supernovae have been proposed to produce those long gamma-ray bursts whose afterglows do not show evidence of association with supernovae (7,8,9).
